Separation membranes such as ultrafiltration membranes and microfiltration membranes are used in various fields such as production of ultrapure water, purification, concentration, and sterilization in pharmaceutical and food industries, and processing of industrial waste water and daily-life waste water. These separation membranes include those of various shapes such as tubular shape, hollow thread shape, and sheet shape. Among these, a sheet-shaped separation membrane reinforced with a porous substrate such as woven cloth or nonwoven cloth has an advantage such that the membrane can be formed even when the strength of the membrane material itself is low.
For this reason, sheet-shaped separation membranes (porous separation membranes) using various membrane materials such as cellulose acetate, polysulfone, polyacrylonitrile, ethylene.vinyl alcohol copolymer, polyvinyl chloride, polyamide, polyimide, and polyvinylidene fluoride are manufactured and used in wide fields by being incorporated into modules of spiral type, plate and frame type, and rotary flat membrane type. Also, a reverse osmosis membrane having a porous separation membrane obtained in this manner as a support membrane is widely used.
A sheet-shaped separation membrane such as described above is manufactured generally by a method referred to as the wet congealing method (non-solvent induced phase separation method). Typically, by this method, a sheet-shaped separation membrane is formed by coating a porous substrate with a membrane-forming solution in which a predetermined polymer is dissolved in a solvent, allowing a micro phase separation to be generated on the liquid membrane surface by moisture in the atmosphere while evaporating the unnecessary solvent, and thereafter immersing into a congealing liquid (see, for example, the patent document 1) or applying a congealing liquid in contact (see, for example, the patent document 2 or 3) thereby to congeal the phase separation layer. Further, in manufacturing hollow thread membranes, the dry-wet type congealing method is known in which a drying step is provided before immersion into the congealing liquid after coating with the membrane-forming solution.
However, when a membrane is formed in a long period of time by such a method, change in the separation membrane ability and defect in the membrane surface are liable to occur with lapse of time by contact of the membrane-forming solution with the congealing liquid or, in the case of immersion, by rise in the solvent concentration or the polymer component concentration in the congealing liquid. In order to solve this problem, a measure such as performing frequent exchange of the congealing liquid or making the congealing liquid tank into a circulation bath has been conventionally tried. However, increase in the waste water processing costs, fluctuation, air bubbles, and concentration unevenness on the liquid surface in the congealing liquid tank, and others have been liable to give adverse effects on the separation membrane ability.
Also, with use of a method of applying the congealing liquid in contact by the bottom-up method or in the lateral direction, the contact with the congealing liquid is liable to be nonuniform depending on the condition of gravity or substrate membrane surface and, when an impurity is caught up, the influence thereof is liable to continue for a long period of time, so that, as a result, the separation membrane ability within the surface is liable to become nonuniform.